So called "insoluble lead or lead alloy" anodes have been used in electrowinning for many years. However, these anodes are slightly soluble and contaminate the electrodeposited metal which is being won. For example, the 84% lead, 14.5% antimony, 0.6% silver alloy anodes used in the Chuquicamata, Chile electrowinning plant, which last from six to eight years in service, have a corrosion rate of 0.6 to 2.5 kilos per ton of copper recovered. At the end of the useful life of these anodes, only 14% of the original weight remains and is recovered as scrap. Most of the lead going into solution in the electrolyte is deposited with the recovered copper. The use of rod type lead anodes and higher anode vs. cathode current density, as described, for example, in the Carosella U.S. Pat. No. 2,766,168 reduces the corrosion rate of the lead based anodes but does not eliminate it.
Likewise when titanium base anodes coated with platinum or platinum group metals are used in electrowinning, the platinum group metals are dissolved and, in addition to the expense of these anodes, the dissolved metals contaminate the metal being deposited on the cathodes.
The use of non-corrodible, dimensionally stable valve metal anode bases, such as titanium, zirconium, hafnium, vanadium, niobium, tantalum and molybdenum having conductive non-passivating surfaces and electrocatalytic properties for oxygen or chlorine discharge at low overvoltages, gives substantial advantages over the graphite, lead, lead alloy, silicon, iron and platinum anodes heretofore used in electrowinning, but to economically use these valve metal anodes for electrowinning, they must be operated at higher current densities than the permissible cathode current densities.